The present invention generally relates to a control unit for controlling a modem, more particularly to a modem which is used for a mobile communication system such as facsimiles used in vehicles, and has an equalizing function, a timing recovery function and a carrier recovery function.
In general, a modem in a receiver demodulates a received signal, which has been modulated in accordance with a PSK (Phase Shift Keying), a DPSK (Differential Phase Shift Keying) or the like, so as to reproduce transmitted data. In a case where the modem demodulates the received signal, a transmission characteristic which affects the transmitted signal in a transmission path, a frequency offset which is the difference between a frequency of a carrier in a transmitter side and a frequency of a carrier in a receiver side, and a carrier phase jitter are respectively corrected. Then a synchronous signal is extracted from the received signal, and thus the transmitted data is reproduced from the received signal in synchronism with the synchronous signal.
The transmission characteristic is corrected by an adaptive equalizer provided in the modem. Coefficients in the adaptive equalizer are updated periodically at a predetermined symbol rate so that a characteristic of the adaptive equalizer cancels the transmission characteristic. The frequency offset and the carrier phase jitter are respectively corrected by a carrier recovery section in the modem. The carrier recovery section rotates a received signal point on a signal constellation by an amount of phase shift which is presumed to correct the frequency offset and carrier phase jitter. The synchronous signal is extracted from the received signal by a timing recovery section in the modem. In a case where the transmitted data is modulated in accordance with PSK, the timing recovery section carries out, for example, the following process so that the synchronous signal is extracted. A bandpass signal (received signal) is multiplied by the carrier and passes through a wave shaping filter so that a complex baseband signal is obtained. Then the complex baseband signal is squared. A synchronous frequency component is extracted from a signal obtained by the above square calculation by a bandpass filter, so that the synchronous signal is obtained. In general, while the received signal is being processed in a case where the synchronous signal is reproduced from the received signal, various coefficients, such as coefficients in the adaptive equalizer, are obtained. Timing information for obtaining a synchronous timing can be extracted from the received signal based on the above various coefficients, and then a sampling period in an analog to digital converter which converts the received signal into digital data can be controlled based on the timing information, so that the transmitted data is reproduced in synchronism with the timing information.
Recently, a modem provided in a mobile communication system, such as a facsimile used in a vehicle, has been examined. The mobile communication system has radio transmission paths. When disturbances are supplied via the radio transmission paths to the modem, functions in the modem can deteriorate. For example, sudden disturbances, such as a short break in the transmitted signal, an amplitude and phase hit and the like, can be generated. Due to these sudden disturbances, the modem has the following trouble. That is, the coefficients in the adaptive equalizer are controlled so as to be diverged, so that the adaptive equalizer malfunctions. In addition, an amount of phase shift which is a presumption error of an amount of phase rotation is increased, and/or the synchronous signal cannot be detected due to timing errors. In these cases, the modem malfunctions, for example, it hangs up.
A high density digital magnetic recording system, such as a DAT (Digital Audio Tape Recorder), also has a problem in that the system malfunctions due to these disturbances. In the high density digital magnetic recording system, when a frequency characteristic of an input signal greatly varies due to scratches, dust and the like, a transmission characteristic of a wave shaping equalizer in the digital magnetic recording system varies. Thus, until the transmission characteristic of the wave shaping equalizer returns to normal, a signal having a distorted wave shape is output from the wave shaping equalizer. As a result, an error rate in a reproduced signal increases. To eliminate the above problems in the digital magnetic recording system, an art disclosed in Japanese Patent Laid Open Publication No. 1-279617 has been proposed.
That is, this art relates to an automatic equalizer in which a signal used in a process for a digital recording of a PCM signal is shaped so that the distortion of the signal is decreased. This automatic equalizer includes a wave shaping equalizing circuit having a transversal type filter, a level detector which detects the level of the input signal, a setting circuit which sets coefficients in the transversal filter based on an input and an output of the wave shaping equalizing circuit, and a control circuit which controls operations in the setting circuit based on the output of the level detector. When the level detector detects that the level of the input signal has become less than a normal level, the control circuit supplies information that the level of the input signal has decreased to the setting circuit. Then the setting circuit stops updating the coefficients in the transversal type filter, so that the coefficients are maintained at predetermined values. Due to the above control operation performed in the automatic equalizer, the characteristic of the automatic equalizer can be prevented from greatly decreasing.
In addition, when the setting circuit is provided with a memory which stores a history of the coefficients in the transversal type filter, the coefficients which are maintained based on the operation of the level detector can be selected from the history stored in the memory. Hence, the coefficients can also be prevented from being set at unusual values based on delay of the detecting operation in the level detector.
However, the prior art related to the digital magnetic recording system can not be applied to the modem in the mobile communication system. That is, since the phase of the carrier in the received signal of the facsimile or the like also varies, the modem cannot be controlled by only the level detector.